1. Field of the Invention
The present invention relates generally to the field of testing electronic devices and more particularly, to a system and method for fault detection of printed circuit boards (PCB's) by imaging their electric field distribution.
2. Description of Prior Art
The testing and repairing of PCBs has become an increasingly complex task. To test and repair circuits, fault isolation measurements must often be made at interior points on a PCB, inaccessible from the PCB's edge connectors or other standard circuit connection points. Direct contact (bed of nails) testers have classically been used to test these internal points, however, such testers impose large localized stresses on boards adversely affecting their lifetime. Additionally, customized nail beds are required for each size and shape of PCB tested. In the case of PCBs with protective layers of plastic, the coating must be destroyed before direct contact testing can be carried out.
As the circuit density and number of printed circuit layers of PCBs increase, the efficiency and practical use of direct contact testers for fault isolation diminishes.
Various methods are known for noninvasive nondestructive technique for PCB testing. One method involves scanning the PCB with an infrared camera. By this method thermal images of working circuit boards may be generated and faults resulting in changes to power distribution are clearly visible. A problem with this method is that any failure which does not result in a change in component temperature cannot be seen. Open signal lines and digital circuit gate failures generally do not change the power dissipation and thus are not detectable by this method.
Another method of noninvasive nondestructive testing, limited to the testing of individual components, utilizes liquid crystals. A single integrated chip is coated with a surfactant solution. A drop of liquid crystal material is placed on the chip. A thin glass cover plate is treated with surfactant solution and lowered onto the drop. Operation of the integrated circuit in the chip causes electric field induced birefringence in the liquid crystal. As a result, a refractive index pattern is produced in the liquid crystal which mirrors the pattern of the electric fields and temperatures at the surface of the integrated circuit. This testing method is described in D. J. Channin, "Liquid-Crystal Technique for Observing Integrated Circuit Operation," IEEE Transactions on Electron Devices, vol. ED-21, pp. 650-652, October 1974, which is incorporated herein.
An apparatus and method is needed for testing an entire PCB which has the benefits of the visualization of component operation provided by the use of liquid crystals and the speed of operation provided by direct contact testers.